Electrochemical devices generally incorporate an electrolyte source to provide the anions or cations necessary to produce an electrochemical reaction. These electrochemical devices include batteries, fuel cells, sensors, electrochemical gas separation systems, electrochromic devices and protein separation devices.
Batteries and fuel cells operate on electrochemical reaction of metal/air, metal/halide, metal/hydride, hydrogen/air, or other materials capable of electrochemical reaction. A zinc/air system, for example, requires the diffusion of hydroxide anions, and typically will incorporate an aqueous potassium hydroxide solution as the electrolyte. The lifetime of this battery is however, limited for several reasons. First, the naked zinc anode is corroded by both the aqueous electrolyte and air. Second, the air channels of the air cathode gradually become blocked by water from the electrolyte solution and third, the electrolyte solution becomes contaminated with zinc oxidation product that diffuses from the anode.
Various methods have been attempted to address the many problems associated with the use of aqueous electrolytes in zinc anode based systems such as zinc/air fuel cells. Additives, for example, have been introduced into the electrolyte solution to extend its lifetime and to protect the anode from corrosion. U.S. Pat. No. 4,118,551 discloses the use of inorganic additives such as mercury, indium, tin, lead, lead compounds, cadmium or thallium oxide to reduce corrosion of a zinc electrode. Many of these additives however, are expensive and more significantly, are very toxic. U.S. Pat. No. 4,378,414 discloses the use of a multi-layer separator between the positive and negative electrodes to reduce corrosion of the anode and contamination of the electrolyte by zinc oxidation products. In addition, hydrophobic materials have been introduced into zinc/air devices to prevent water permeation into the air channels of the cathode. Introduction of hydrophobic materials is however, a difficult process and may result in decreased performance of the cathode.
In addition to zinc/air systems, other metal/air systems, such as aluminum/air, lithium/air, cadmium/air, magnesium/air, and iron/air systems, also have the potential for many different applications due to their theoretically high ampere-hour capacity, voltage, and specific energy. In actual practice however, these very promising theoretical values are greatly reduced due to the corrosion of the metal anode in the electrolyte.
A solid-state hydroxide conductive electrolyte polybenzimidazole (“PBI”) film is disclosed in U.S. Pat. No. 5,688,613 and comprises a polymeric support structure having an electrolyte active species dispersed therein, wherein the polymer structure is in intimate contact with both the anode and the cathode. This PBI film, however, does not absorb water and therefore, does not hold water within the membrane, causing it to dry out quickly.
U.S. Pat. No. 3,871,918 discloses an electrochemical cell embodying an electrode of zinc powder granules suspended in a gel comprised of methylenebisacrylamide, acrylic acid and acrylamide. Potassium hydroxide serves as the electrolyte, and is contained within the gel.
With regard to devices that rely on the conduction of cations, while there has been a significant amount of research in this area, most proton conducting membranes are very expensive to produce and typically do not function at room temperature. In the 1970's for example, a fully fluorinated polymer membrane, NAFION® (DuPont, Wilmington, Del. USA) was introduced and has served as the basis from which subsequent proton conducting membranes have evolved.
U.S. Pat. No. 5,468,574 discloses a proton conductive membrane that is characterized as a highly sulfonated polymeric membrane composed of block copolymers of sulfonated polystyrene, ethylene and butylene blocks. In 1997, NASA's Jet Propulsion Laboratory disclosed the development of an improved proton conductive membrane composed of sulfonated poly(ether ether ketone), commonly known as H-SPEEK.
The separator in a cell or battery physically separates and electrically insulates electrodes of different polarity. While serving as a barrier to the transport of active materials of the different electrodes, a separator should provide ionic conduction. Good ionic conductivity is necessary to ensure that an electrochemical cell/battery is capable of delivering usable amounts of power for a given application.
In a rechargeable electrochemical cell, a separator is also used to prevent short-circuiting, caused by metal dendrite penetration during recharging. For example, in rechargeable zinc/air cells, zinc on the surface of the negative zinc electrode (anode) is dissolved as zincate ion into the electrolyte solution during discharge. Then, during the charge, when the charging current is typically below 20 mA/cm2, depending on the particular anode used, the zincate ion forms dendritic zinc, which is needle-like and grows from the negative electrode toward the charging electrode. Unfortunately, these needle-like structures can pierce through conventional separators causing an internal short circuit. The service life of the cell is consequently terminated. In addition to preventing dendrite penetration, the separator must allow for the exchange of electrolytic ions during both discharging and charging of the cell.
The most commonly used separators in rechargeable cells are porous insulator films of polyolefins, polyvinyl alcohol (PVA), nylon, or cellophane. Acrylic compounds may also be radiation-grafted onto these separators to make them more wettable and permeable to the electrolyte. Although much work has been done to improve the performance of separators, dendrite penetration problems are frequently encountered with these and other conventional separators, as well as problems involving diffusion of reaction products such as the metal oxide to remaining parts of the cell.
With conventional separators, controlling the pore size of the separator is the only effective way to avoid dendrite penetration and prevent product diffusion. By doing this, however, the ionic conductivity of the separator is also greatly reduced. This creates a bottleneck for high charging-discharging current density operations, important considerations for use in some applications, such as in electrical vehicles.
U.S. Pat. No. 5,549,988 (the '988 patent) discloses an electrolyte system separator disposed between the cathode and anode of a rechargeable electrochemical battery. The electrolyte system includes a polymer matrix prepared from polyacrylic acid or derivatives thereof. An electrolyte species, such as KOH or H2SO4, is then added to the polymer matrix to complete the system. However, as reported in the '988 patent, the measured ionic conductivities of the disclosed electrolyte-polymer films are low, ranging from 0.012 S/cm to 0.066 S/cm. Although these conductivities are acceptable for some applications, they are inadequate for other high rate operations including electrical vehicles.
An electrochemical reaction is also involved in the function of electrochromic devices (ECD's). Electrochromism is broadly defined as a reversible optical absorption change induced in a material by an electrochemical redox process. Typically, an electrochromic device contains two different electrochromic materials (ECM's) having complementary properties; the first is generally reduced, undergoing a color (1)-to-color (2) transition during reduction, while the second material is oxidized, undergoing a similar transition upon the loss of electrons.
Basically, there are two types of electrochromic devices, depending upon the location of the electrochromic materials within the device. In a thin-film type device, the two ECM's are coated onto the two electrodes and remain there during the redox coloration process. In a solution-phase device, both ECM's are dissolved in an electrolyte solution and remain their during the coloration cycle. The solution-phase device is typically more reliable and has a longer lifetime, however, in order to maintain the colored state, an external power source must be continuously applied. As the thin-film type device does not need an external power source to maintain its colored state, power consumption is greatly reduced, making this an advantage for such energy-saving applications as smart windows. The drawback of the thin-film type device is that it has a short lifetime. After a certain number of cycles, ECM films can lose contact with the electrode, or they may no longer be capable of phase change and the device expires.
With regard to solution-phase devices, U.S. Pat. No. 5,128,799, for example, discloses a method of reducing the current required to maintain the colored state that involves the addition of gel into the device. While reducing energy consumption however, the addition of the gel into the device also greatly reduces the switching speed of the device. With regard to thin-film devices, attempts to extend the lifetime of the device have included changes to the crystal structure of the film. While such changes have increased the lifetime of thin-film devices to an extent, the typical lifetime of such devices is still not satisfactory.
The foregoing problems thus present major obstacles to the successful development and commercialization of fuel cell technology, a green energy source, and of electrochromic devices such as smart windows and flat panel displays, which have several energy-saving, decorative, and information display applications. With respect to the problems associated with rechargeable electrochemical cells, it is clear that there is a great need for a separator that can provide improved ionic conductivity while providing an effective barrier against the penetration of metal dendrites and the diffusion of reaction products.